Platen cleaning method

ABSTRACT

A method for cleaning a workpiece support that includes using a workpiece that has been coated on its bottom surface with a suitable material is disclosed. This specially coated workpiece is placed on the support, and some time later, it is removed, taking with it particles from the support. In certain embodiments, the workpiece undergoes an ion implantation process to increase its temperature, and to increase the tackiness of the coating on the bottom surface. The material used to coat the bottom can be of variable types, including photoresists, oxides and deposited glasses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 12/243,980, filedOct. 2, 2008, the disclosure of which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

Ion implanters are commonly used in the production of semiconductorwafers. In some embodiments, an ion source is used to create an ionbeam, which is then directed toward the wafer. As the ions strike thewafer, they dope a particular region of the wafer. The configuration ofdoped regions defines their functionality, and through the use ofconductive interconnects, these wafers can be transformed into complexcircuits. In other embodiments, ion implantation is performed byexposing a wafer directly to a plasma, a technique called “plasmaimmersion implantation.”

A block diagram of a representative ion implanter 100 is shown inFIG. 1. An ion source 110 generates ions of a desired species. In someembodiments, these species are atomic ions, which may be best suited forhigh implant energies. In other embodiments, these species are molecularions, which may be better suited for low implant energies. These ionsare formed into a beam, which then passes through a source filter 120.The source filter is preferably located near the ion source. The ionswithin the beam are accelerated/decelerated in column 130 to the desiredenergy level. A mass analyzer magnet 140, having an aperture 145, isused to remove unwanted components from the ion beam, resulting in anion beam 150 having the desired energy and mass characteristics passingthrough resolving aperture 145.

In certain embodiments, the ion beam 150 is a spot beam. In thisscenario, the ion beam passes through a scanner 160, which can be eitheran electrostatic or magnetic scanner, which deflects the ion beam 150 toproduce a scanned beam 155-157. In certain embodiments, the scanner 160comprises separated scan plates in communication with a scan generator.The scan generator creates a scan voltage waveform, such as a sine,sawtooth or triangle waveform having amplitude and frequency components,which is applied to the scan plates. In a preferred embodiment, thescanning waveform is typically very close to being a triangle wave(constant slope), so as to leave the scanned beam at every position fornearly the same amount of time. Deviations from the triangle are used tomake the beam uniform. The resultant electric field causes the ion beamto diverge as shown in FIG. 1.

In an alternate embodiment, the ion beam 150 is a ribbon beam. In suchan embodiment, there is no need for a scanner, so the ribbon beam isalready properly shaped.

An angle corrector 170 is adapted to deflect the divergent ion beamlets155-157 into a set of beamlets having substantially paralleltrajectories. Preferably, the angle corrector 170 comprises a magnetcoil and magnetic pole pieces that are spaced apart to form a gap,through which the ion beamlets pass. The coil is energized so as tocreate a magnetic field within the gap, which deflects the ion beamletsin accordance with the strength and direction of the applied magneticfield. The magnetic field is adjusted by varying the current through themagnet coil. Alternatively, other structures, such as parallelizinglenses, can also be utilized to perform this function.

Following the angle corrector 170, the scanned beam is targeted towardthe workpiece 175. The workpiece is attached to a workpiece support. Theworkpiece support provides a variety of degrees of movement.

An alternative embodiment of an ion implantation system, plasmaimmersion, is shown in FIG. 2. The plasma doping system 200 includes aprocess chamber 202 defining an enclosed volume 203. A platen 234 may bepositioned in the process chamber 202 to support a workpiece 238. In oneinstance, the workpiece 238 may be a semiconductor wafer having a diskshape, such as, in one embodiment, a 300 millimeter (mm) diametersilicon wafer. The workpiece 238 may be clamped to a flat surface of theplaten 234 by electrostatic or mechanical forces. In one embodiment, theplaten 234 may include conductive pins (not shown) for making connectionto the workpiece 238.

A gas source (not shown) provides a dopant gas to the interior volume103 of the process chamber 202. A gas baffle 270 is positioned in theprocess chamber 202 to deflect the flow of gas from the gas source. Apressure gauge (not shown) measures the pressure inside the processchamber 202. A vacuum pump 212 evacuates exhausts from the processchamber 202 through an exhaust port 210 in the process chamber 202. Anexhaust valve 214 controls the exhaust conductance through the exhaustport 210.

The process chamber 202 may have a chamber top 218 that includes a firstsection 220 formed of a dielectric material that extends in a generallyhorizontal direction. The chamber top 218 also includes a second section222 formed of a dielectric material that extends a height from the firstsection 220 in a generally vertical direction. The chamber top 218further includes a lid 224 formed of an electrically and thermallyconductive material that extends across the second section 222 in ahorizontal direction.

The plasma doping system may further include a source 201 configured togenerate a plasma 240 within the process chamber 202. The source 201 mayinclude a RF source 250, such as a power supply, to supply RF power toeither one or both of the planar antenna 226 and the helical antenna 246to generate the plasma 240. The RF source 250 may be coupled to theantennas 226, 246 by an impedance matching network 252 that matches theoutput impedance of the RF source 250 to the impedance of the RFantennas 226, 246 in order to maximize the power transferred from the RFsource 250 to the RF antennas 226, 246.

The plasma doping system 200 also may include a bias power supply 248electrically coupled to the platen 234. The bias power supply 248 isconfigured to provide a pulsed platen signal having pulse ON and OFFtime periods to bias the platen 234, and, hence, the workpiece 238, andto accelerate ions from the plasma 240 toward the workpiece 238 duringthe pulse ON time periods and not during the pulse OFF periods. The biaspower supply 248 may be a DC or an RF power supply.

In operation, the gas source (not shown) supplies a primary dopant gascontaining a desired dopant for implantation into the workpiece 238. Agas pressure controller regulates the rate at which the primary dopantgas is supplied to the process chamber 202. The source 201 is configuredto generate the plasma 240 within the process chamber 202. To generatethe plasma 240, the RF source 250 resonates RF currents in at least oneof the RF antennas 226, 246 to produce an oscillating magnetic field.The oscillating magnetic field induces RF currents into the processchamber 202. The RF currents in the process chamber 202 excite andionize the primary dopant gas to generate the plasma 240.

The bias power supply 248 provides a pulsed platen signal to bias theplaten 234 and, hence, the workpiece 238 to accelerate ions from theplasma 240 toward the workpiece 238 during the pulse ON periods of thepulsed platen signal. The frequency of the pulsed platen signal and/orthe duty cycle of the pulses may be selected to provide a desired doserate. The amplitude of the pulsed platen signal may be selected toprovide a desired energy. With all other parameters being equal, agreater energy will result in a greater implanted depth.

The workpiece support is used to both hold the wafer in position, and toorient the wafer so as to be properly implanted by the ion beam or byplasma immersion. To effectively hold the wafer in place, most workpiecesupports typically use a circular surface on which the workpiece rests,known as a platen. Often, the platen uses electrostatic force to holdthe workpiece in position. By creating a strong electrostatic force onthe platen, also known as the electrostatic chuck, the workpiece orwafer can be held in place without any mechanical fastening devices.This minimizes contamination and also improves cycle time, since thewafer does not need to be unfastened after it has been implanted. Thesechucks typically use one of two types of force to hold the wafer inplace: coulombic or Johnsen-Rahbek force.

The equipment described above is typically located within a very lowpressure, preferably a near vacuum, environment. Workpieces, such aswafers, are introduced into the vacuum environment via a FOUP (FrontOpening Unified Pod). Each wafer is placed on the workpiece support,implanted with ions and then removed from the environment.

Large numbers of wafers are processed in this manner. The highthroughput of wafers introduces particles into the workpieceenvironment, and specifically onto the workpiece support. While theseparticles are very small, usually in the 0.09 to 2 micron in diameter,they can still negatively impact the implantation process. Inparticular, particles accumulated on the platen can be transferred tothe back of the wafer. While no devices are constructed on the back ofthe wafer, nevertheless these backside particles have two adverseeffects on future process steps. Firstly, backside particles on onewafer may transfer to the frontside of a neighboring wafer in any batchprocess, or even in the FOUP. Secondly, larger backside particles (0.5to 2 micron) elevate a local portion of the wafer above the plane offocus during lithography causing misfocus and hence yield loss throughpoor pattern definition.

Thus, it is customary to periodically clean the workpiece support. Thisprocedure typically entails venting the chamber so that it returns tonormal atmospheric pressure, then cleaning the workpiece support. Thiscleaning process may be mechanical in nature, such as rubbing thesupport, or may be chemical, such as using a mist spray. The chamber isthen returned to vacuum conditions and the ion implantation process canproceed.

Unfortunately, this procedure is very time consuming, often taking hoursof valuable time to return the process chamber to vacuum conditions.During this recovery time, the chamber is idle, thereby lowering theefficiency of the system, and increasing cost of ownership. Furthermore,exposure of the process chamber to atmosphere has other undesirableeffects: moisture in the air can delaminate built-up films on theprocess chamber walls and other surfaces, which generates flaking andleads to frontside particles. Thus, venting the process chamberindirectly leads to longer recovery time than the vent and pumpprocesses alone.

A method of cleaning the workpiece support which does not require thelong recovery times associated with present day cleaning procedureswould be extremely beneficial.

SUMMARY OF THE INVENTION

The problems of the prior art are overcome by the workpiece supportcleaning method described in the present disclosure. The disclosureprovides a method for cleaning a workpiece support, such as a platen,that includes using a workpiece, such as a semiconductor wafer, that hasbeen coated on its bottom surface with a material suitable for removingparticles from the workpiece support. This workpiece is placed on thesupport, and some time later, it is removed, taking with it particlesfrom the support. In certain embodiments, the workpiece undergoes aprocess such as, but not limited to ion implantation, to increase itstemperature and to increase the tackiness of the coating material on thebottom surface. The material used to coat the bottom can be of variabletypes, including photoresists, oxides and deposited glasses.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 represents a traditional ion implanter; and

FIG. 2 represents a plasma immersion ion implanter.

DETAILED DESCRIPTION OF THE INVENTION

As stated above, the amassing of particles on the workpiece support,such as a platen, can be detrimental to the ion implantation process.The time required cleaning these particles from the workpiece supportand cost of ownership is substantially reduced through the use of thein-situ cleaning method disclosed herein.

A workpiece having a coating of a suitable material deposited on itsbottom surface is brought into contact with the workpiece support, suchas the electrostatic chuck. In the preferred embodiment, this workpieceis a semiconductor wafer, which is loaded into the vacuum chamber viathe traditional entry path using a FOUP. In this way, the coating isbrought into contact with not only the chuck, but also other surfaces ofthe wafer handling system.

In one embodiment, coatings such as deposited glasses, doped glasses andnitrides may be employed. These coatings may be slightly tacky. In otherembodiments, the coating is softer than the surface that it is intendedto clean. In other embodiment, the coating may retain electrostaticcharge.

In another embodiment, the wafer is coated on the bottom surface withphotoresist. Photoresist is commonly used as part of the lithography (orpattern forming) process during semiconductor wafer fabrication.Photoresist is typically categorized as either hard-baked or soft-bakedand either of these types is suitable for the present application.

In one specific embodiment, photoresist, such as is available fromShipley Company, is used. In a further embodiment, Shipley AZphotoresist is used. First, the wafer is cleaned with the nitrogen gasblower turned off. The photoresist is then applied to the wafer using aphotoresist spinner. The application of photoresist is preferablyperformed one wafer at a time. The wafer is then baked in a convectionoven at about 110° C. for about 1 minute. This baking can be performedas a batch process. This soft-baked wafer is then placed in the FOUP andused to clean the platen as described above.

In a further embodiment, after baking at 110° C., the wafer is bakedagain at 130°-150° C., to create a hard-baked photoresist coating. Thewafer is then entered into the FOUP as described above.

In another embodiment, an oxide, such as silicon oxide is used. In afurther embodiment, the silicon oxide is thermally grown on the bottomsurface of the wafer.

As mentioned above, in the preferred embodiment, the specially coatedwafer enters the vacuum chamber via the FOUP. It then is moved to theworkpiece support. Between the FOUP and the workpiece support, the waferis typically contacted by one of more pieces of equipment, such asrobotic arms. As the wafer is passed, the special coating removesparticles from each piece of equipment it contacts. When the waferreaches the workpiece support, or chuck, it is clamped down. In mostembodiments, this clamping is performed electrostatically, but can alsobe performed mechanically through the use of clamps or other securingmechanisms.

The act of clamping the wafer to the chuck brings them in close contactand enhances the transfer of particles from the surface of the chuck tothe bottom side of the wafer. In certain embodiments, the ionimplantation process is performed on this specially coated wafer. Byimplanting ions in the wafer, its temperature increases, which increasesthe stickiness of the coating. In certain embodiments, the ionimplantation is performed using a specific recipe of dose and energy. Inother embodiments, the implantation only serves to elevate thetemperature of the wafer sufficiently to warm the coating on the bottomsurface. In other embodiments, the wafer is warmed using other methods,such as heat lamps. In still other embodiments, one can modulate theclamping force or clamping sequence.

The wafer is then removed from the support, and exits the vacuum chamberin the same manner employed by standard semiconductor wafers. In certainembodiments, the surface of the chuck is best cleaned by repeating theabove process a number of times. In some embodiments, 12 or more wafersare utilized, although other numbers are also within the scope of thedisclosure.

While the embodiments described herein are used in conjunction with ionimplantation equipment, the disclosure is not limited to thisembodiment. For example, the methods described herein are equallysuitable for Plasma Enhanced Chemical Vapor Deposition (PECVD), etch,Physical Vapor Deposition (PVD), and lithography.

1. A method of removing particles from a surface of a workpiece supportused to hold a workpiece comprising: coating a bottom surface of saidworkpiece with an oxide; placing said bottom surface of said workpieceonto said workpiece support; clamping said workpiece onto said workpiecesupport; and removing said workpiece.
 2. The method of claim 1, whereinsaid workpiece comprises a semiconductor wafer.
 3. The method of claim1, wherein said clamping is performed electrostatically.
 4. The methodof claim 1, wherein said clamping is performed mechanically.
 5. Themethod of claim 1, further comprising heating said workpiece after saidclamping.
 6. The method of claim 5, wherein said heating comprises usingheat lamps.
 7. The method of claim 5, wherein said heating comprisingimplanting ions into said workpiece.
 8. The method of claim 1, whereinsaid workpiece is used as part of an ion implantation process.
 9. Themethod of claim 8, wherein said ion implantation process utilizes an ionbeam.
 10. The method of claim 8, wherein said ion implantation processutilizes plasma immersion.
 11. The method of claim 1, wherein saidworkpiece is used as part of a process selected from the groupconsisting of plasma enhanced chemical vapor deposition, physical vapordeposition, lithography and etching.
 12. The method of claim 1, furthercomprising transferring particles from said workpiece support to saidbottom surface of said workpiece prior to said removing.